U.S. Pat. Nos. 8,684,953 and 9,138,566, to the present inventor, describe steering tools for steering medical devices through body lumens. The steering tool has an internal tube disposed inside an external tube. The internal and external tubes are arranged for longitudinal axial movement relative to one another. The distal end of the internal tube is fixedly joined to the distal end of the external tube. One or both of the internal and external tubes is slotted near the distal end thereof. The longitudinal axial movement causes bending of the distal ends of the tubes. The steering tool provides a distal tip which combines steerability, flexibility and torqueability. The tool eliminates the need for pull/push wires.
It is important for the tube to have sufficient flexibility to enable pushing the steering tool (or also called steering catheter) inside a body lumen like a blood vessel. However, it is also important that the distal end of the steering tool tracks or follows the manipulating movements of the operator at the proximal end of the tool (“trackability”). If the tool is too flexible the distal end will not respond correctly to the manipulating movements and will not properly track or follow the desired motion.
The mechanical properties of the tool material and the tool dimensions, such as but not limited to, the elasticity modulus, bending strength, shear strength, tube thickness, tube diameter, moments of inertia, and others, will determine the flexibility and trackability of the tool. Typical materials for such tubes include stainless steel alloys and nitinol, which can reduce the wall thickness, but often these tubes are too strong for bending through tortuous bends in body lumens.
Cutting the tube in a variety of shapes may improve the flexibility and trackability of the tool. However, there are disadvantages and compromises. For example, cutting the material reduces the ability to push the tube through different bends because the tube can collapse prematurely before going fully through the bend. In addition, cutting the material can increase tube elongation, create detrimental torque effects, or can create local distortions while pushing and pulling. For example, if the tube is cut so it has a shape like a spring; pushing and pulling the tube can generate rotation and unpredictable tip movements.
FIGS. 1A and 1B illustrate one prior art cutting pattern—a homogeneous spiral cutting pattern (which is one type of homogeneous spring shape). This pattern may work well with pulling, but has length changes during pushing and can generate undesirable and random rotation at the tip. Another disadvantage is that the spiral cutting transfers torque in one rotational direction (e.g., clockwise) but is limited and elongated in the opposite rotational direction (e.g., counterclockwise).
One possible solution is to connect the spiral members with stiffeners, as shown in FIGS. 2A and 2B. This is an improvement because the configuration is stiffer and less elongated in the pulling direction. However, the spiral members between the connections act like short springs. The result is this configuration still suffers from local rotations at each of the short springs; the accumulation of all the short springs can add up to a significant rotation. Another drawback is this configuration also responds differently to clockwise rotation as opposed to counterclockwise rotation.
Another possible solution is to use orthogonal connectors, as shown in FIGS. 3A and 3B and in FIGS. 4A and 4B. The connectors are perpendicular (orthogonal) to the connectors of the previous row. The cutting shape is repeated in any second row. Similar patterns may be used with greater numbers of connectors but the overall catheter stiffness would be greater.
Orthogonal cutting may be used to make a minimal number and size of connections for good flexibility. The number of cuts is limited by the tube thickness and diameter and the tube material properties. In order to achieve good tube flexibility in all directions, the cuts are shifted with respect to each other. In order to maintain a one-to-one rotational transfer from the proximal end to the distal end the cuts create identical moments of inertia all around the perimeter of the tube at any axial location on the tube.
However, the pattern consistently and continuously repeats itself in a spiral manner along the axial direction of the tube and this creates the undesirable spring effect. This is indicated by arrows 3 and 4, respectfully, in FIGS. 3A and 4A.